1. Field of the Invention
The present invention generally relates to lithography, more specifically lithography for semiconductor processing.
More specifically, this invention relates to a method for measuring contamination of a lithographical element. This invention also relates to a system for measuring contamination of a lithographical element.
2. Description of the Related Technology
Optical lithography nowadays uses wavelengths of 248 nm or 193 nm. With 193 nm immersion lithography integrated circuit (IC) manufacturing is possible down to 45 nm or even down to 32 nm node. However for printing in sub-32 nm half pitch node, this wavelength is probably not satisfactory due to theoretical limitations, unless double patterning is used. Instead of using wavelengths of 193 nm, a more advanced technology has been introduced, also referred to as extreme ultraviolet lithography (EUV lithography), which uses wavelengths of 10 nm to 14 nm, with a typical value of 13.5 nm. This technique was previously also known as soft X-ray lithography more specifically using wavelengths in the range of about 2 nm to 50 nm.
In optical lithography at some wavelengths in the deep ultra violet (DUV) range, the electromagnetic radiation is transmitted by most materials, including glass used for conventional lenses and masks.
At short wavelengths however, e.g. for extreme ultraviolet lithography and soft X-ray lithography, the electromagnetic radiation is absorbed by most materials, including glass used for conventional lenses and masks. Therefore a completely different tool is necessary for performing EUV lithography compared to conventional optical lithography. Instead of using lenses, such an imaging system presently relies on all-reflective optics and therefore is composed of reflective optical elements, also referred to as catoptric elements, for example mirrors. These reflective optical elements, e.g. mirrors preferably are coated with multilayer structures designed to have a high reflectivity (up to 70%) at the 13.5 nm wavelength. Furthermore, since air will also absorb EUV light, a vacuum environment is necessary.
Although EUV lithography is considered applicable using wavelengths less than 32 nm, still a lot of problems need to be overcome to reach a mature technology. As presented in a publication of K. R. Dean et al in Proc. of SPIE 6153E, p. 1-9 (2006), one of these problems is contamination of the optics by chemical components, also referred to as “contamination”, which components are usually gaseous components originating from outgassing of the resist. This resist outgassing occurs due to the EUV irradiation of the EUV resist. It was noticed that at the regions where radiation falls on the EUV optics, the components contamination that outgases during the exposure may contaminate the EUV optics and as a consequence diminishes both the reflectivity of the reticle as well as the reflectivity of the imaging optics. As a consequence the lifetime of the EUV optics is reduced seriously due to contamination. Following the International Technology Roadmap for Semiconductors (ITRS) the organic material outgassing rate for 2 minutes under the lens should be lower than 5e13 molecules/cm2-sec.
In order to reduce the resist outgassing rate, metrology tools are necessary which are able to measure the amount of resist outgassing for certain resists. One possibility for screening resist outgassing is also described in a publication of K. R. Dean et al in Proc. of SPIE 6153E, p. 1-9 (2006). An outgassing chamber is built on a synchrotron beam line. Together with the wafers, a Si3N4 witness plate is put in the chamber and exposed to EUV irradiation. This witness plate is then analyzed with electron spectroscopy for chemical analysis (ESCA) to find evidence of the contamination build up. The contaminants are collected in thermal desorption (TD) tubes. The contaminants in these TD tubes are analyzed by gas chromatography/mass spectroscopy (GC/MS) for chemical analysis.
US 2003/0011763 describes a projection exposure apparatus useful for projection exposure of a pattern defined on a mask onto a substrate in the manufacture of a semiconductor device. The apparatus comprises a cleaning device for cleaning an optical member. Contamination of a substrate-opposed surface of an optical member at the time of pattern transfer can be reduced by, prior to pattern transfer, removing the contaminants caused by the previous pattern transfer by a cleaning operation. In embodiments of US 2003/0011763, the cleaning operation may be performed only when a numerical value for a contamination level which is determined based on a difference between a predetermined reflectance and an actually determined reflectance of the optical member is out of a certain permissible range. The predetermined reflectance is determined immediately after the apparatus is manufactured.
US 2005/0083515 describes a method for evaluating reflection uniformity of an optical component having EUV reflective surface for use in EUV lithography. The method may be used to determine coating and substrate induced reflectivity losses of a substrate. According to embodiments of US 2005/0083515 a reflectivity map of a test piece is compared to a reference piece of identical design that has been independently characterized.
Other techniques and systems still are required to analyze resist outgassing, with a special interest for in-situ measurement of the contamination.